Since a photographic process using silver halide is excellent in photographic properties such as sensitivity and gradation control as compared to other photographic processes, such as, for example, an electrophotographic process and a diazo photographic process, the silver halide photographic process has been most widely used. Recently, however, a technique capable of easily and rapidly obtaining images by employing a dry process such as heating, etc., as the image-forming process of a light-sensitive material using silver halide in place of a conventional wet process by a developing solution, etc., has been developed.
Heat developable light-sensitive materials are known in the field of the art and heat developable light-sensitive materials and processes for processing these light-sensitive materials are described in, for example, "Shashin Kogaku no Kisa (The Basis of Photographic Engineering)", pages 553-555, published by Corona K. K., 1979; "Eizo Joho (Image Information)", page 40, published April, 1978; "Nebletts Handbook of Photography and Reprography", 7th Ed, pages 32-33, published by Van Nostrand Reinhild Company; U.S. Pat. Nos. 3,152,904; 3,301,678; 3,392;020; 3,457,075; U.K. Pat. Nos. 1,131,108; 1,167,777; and "Research Disclosure", June 1978, pages 9-15 (RD-19029).
Various processes of obtaining color images by dry systems have been proposed. For example, for a process of forming color images by the combination of the oxidation product of a developing agent and couplers, there are provided p-phenylenediamine reducing agents and phenolic or active methylene couplers in U.S. Pat. No. 3,531,286; p-aminophenol reducing agents in U.S. Pat. No. 3,761,270; sulfonamidophenol reducing agents in Belgian Pat. No. 802,519 and "Research Disclosure", September 1975, pages 31 and 32; and the combination of sulfonamidophenol reducing agents and 4-equivalent couplers in U.S. Pat. No. 4,021,240.
Also, a process of introducing a nitrogen-containing heterocyclic group into a dye, forming a silver salt, and liberating the dye by heat development in the presence of the silver salt is described in "Research Disclosure", May 1978, pages 54-58 (RD-16966). However, in the process, clear images cannot be obtained since it is difficult to control the liberation of dye at unexposed areas and hence the process is unsuitable for general use.
Furthermore, for a process of forming positive color images by a heat-sensitive silver dye bleaching process, useful dyes and bleaching processes are described in, for example, "Research Disclosure", April 1976, pages 30-32 (RD-14433); ibid., December 1976, pages 14-15 (RD-15227); U.S. Pat. No. 4,235,957, etc.
However, the foregoing processes have the faults that an additional step of superposing an activating agent sheet followed by heating is required for accelerating the bleaching of the dye and also the color image formed is gradually bleached by reduction with existing free silver during the preservation of the color image for a long period of time.
Also, a process of producing color images utilizing leuco dyes is described in, for example, U.S. Pat. Nos. 3,985,565 and 4,022,617. However, the process has a fault that it is difficult to stably retain leuco dyes in light-sensitive materials and hence the light-sensitive material is gradually colored during the preservation thereof.
Furthermore, the foregoing various processes have the faults that a relatively long period of time is generally required for development and images obtained have high fog and low density.